Base station midamble selection

ABSTRACT

Midamble codes are selected in a wireless time division duplex communication system. At least one channelization code is assigned to a user equipment. A mapping is provided between midamble codes and channelization codes. At least one midamble code is mapped to a plurality of channelization codes having a same spreading factor. The midamble code mapped to the at least one channelization code to the user equipment is selected. A communication burst is formed with the assigned at least one channelization code and the selected midamble code.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/090,498, filed Mar. 4, 2002 which is a continuation of U.S. patentapplication Ser. No. 09/775,969, filed Feb. 2, 2001, which claimspriority from U.S. Provisional Patent Application No. 60/180,402, filedFeb. 4, 2000, all of which are incorporated by reference as if fully setforth.

BACKGROUND OF THE INVENTION

The present invention relates generally to wireless time division duplexusing code division multiple access (TDD/CDMA) communication systems. Inparticular, the invention relates to determining channelization codesfor use in multiuser detection in the downlink for TDD/CDMA systems.

A TDD/CDMA communication system is illustrated in FIG. 1. The system 10has multiple base stations 12 ₁ to 12 ₅. Each base station 12 ₁ has anassociated operating area. User equipments (UEs) 14 ₁ to 14 ₃ in a basestation's operating area communicate with that base station 12 ₁.Communications transmitted from a base station 12 ₁ to a UE 14 ₁ arereferred to as downlink communications and communications transmittedfrom a UE 14 ₁ to a base station 12 ₁ are referred to as uplinkcommunications.

In a wireless TDD/CDMA communication system, multiple communications aresent in a shared frequency spectrum. One such system is proposed in athird generation wideband-CDMA (W-CDMA) standard. In CDMA systems,multiple communications are sent in the shared spectrum and aredistinguished by channelization codes. In TDD/CDMA systems, the sharedspectrum is also time divided using repeating frames having a fixednumber of time slots, such as fifteen (15) time slots. Each time slot isused to transmit either only uplink or downlink communications. As aresult, the communications are distinguished by both channelizationcodes and time slots. A single channelization code used in a single timeslot is referred to as a resource unit. Based on a communicationsbandwidth, the communication may require one or multiple resource units.Typical data modulation schemes used in TDD/CDMA systems are quadraturephase shift keying (QPSK), binary phase shift keying (BPSK) and NQuadrature Amplitude Modulation (QAM), such as N=8, 16 or 64.

Data is transmitted in such systems using communication bursts 16. Acommunication burst 16 carries data in a single time slot using a singlechannelization code (a single resource unit). A typical communicationburst 16 has a midamble 20, a guard period 18 and two data bursts 22,24, as shown in FIG. 2. The midamble 20 separates the two data bursts22, 24. The guard period 18 separates the communication bursts 16 toallow for the difference in arrival times of bursts 16 transmitted fromdifferent transmitters. The two data bursts 22, 24 contain thecommunication burst's data. The midamble 20 contains a midamble code foruse in estimating the channel response between the receiver andtransmitter.

Since multiple communication bursts may be transmitted in a single timeslot, a receiver must be able to distinguish data from the multiplebursts. One approach to recover the received data is multiuser detection(MUD).

In MUD, a receiver recovers all communication bursts' data in a timeslot, including bursts transmitted to other UEs. To recover all thebursts' data, the MUD receiver needs to know all of the channelizationcodes used to transmit the bursts. In the proposed TDD mode of W-CDMA,each UE 14 ₁ to 14 ₃ only knows which channelization and midamble codesare used for carrying information intended for it. To determine all thechannelization and midamble codes, a bank of matched filters is used todetect all possible channelization/midamble combinations. The outputpower from each matched filter is compared to a threshold to determinewhether a particular channelization/midamble combination was used. Dueto the number of required matched filters, this approach has a highcomplexity. Additionally, if there is a high correlation betweenchannelization codes, this approach may have poor performance.Accordingly, it is desirable to have alternate approaches for UEs 14 ₁to 14 ₃ to be able to determine the active channelization codes.

SUMMARY OF THE INVENTION

Midamble codes are selected in a wireless time division duplexcommunication system. At least one channelization code is assigned to auser equipment. A mapping is provided between midamble codes andchannelization codes. At least one midamble code is mapped to aplurality of channelization codes having a same spreading factor. Themidamble code mapped to the at least one channelization code to the userequipment is selected. A communication burst is formed with the assignedat least one channelization code and the selected midamble code.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a time division duplex/code divisionmultiple access communication system.

FIG. 2 is an illustration of a communication burst.

FIG. 3 is an illustration of a simplified base station transmitter and auser equipment receiver.

FIG. 4 is a flow chart of downlink channelization code identification.

FIG. 5 is an illustration of midamble sequence to channelization codemapping.

FIG. 6 is a channelization code detection device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 3 illustrates a simplified base station transmitter 26 and a UEreceiver 28 using multiuser detection (MUD). Data to be communicated tothe active UEs 14 ₁ to 14 ₃ is produced by data generators 32 ₁ to 32_(K). Each generator 32 ₁ to 32 _(K) produces data to be sent in aparticular communication burst. Each communication burst's generateddata is subsequently formatted into a communication burst by a spreadingand modulation device 34 ₁ to 34 _(K). The spreading and modulationdevice 34 ₁ to 34 _(K) adds the midamble and spreads the generated datawith a channelization code associated with that communication burst.Additionally, the spread data is time multiplexed into the appropriatetime slot. All of the communication bursts are combined by a combiner52. The combined communication bursts are modulated up to radiofrequency, such as by a mixer 36, and the radio frequency signal isradiated by an antenna 38 through a wireless radio channel 30. Iftransmit diversity is utilized by the base station 14 ₁, the radiofrequency signal will be transmitted by multiple antennas.

At a UE receiver 28, radio frequency signals are received by an antenna40. The received signals are demodulated to a baseband signal, such asby a mixer 42. A channel estimation device 44 is used to estimate thechannel that the communication bursts were transmitted in using thetransmitted midamble codes. A multiuser detection (MUD) device 46processes the baseband signal using the estimated channel informationand the active channelization codes to produce hard symbols.

Identifying active channelization codes is shown in the flow chart ofFIG. 4. One approach to aid in identifying active channelization codesat the UE 14 ₁ is to provide a mapping between midamble codes (midamblesequences) 54 ₁ to 54 _(N) and channelization codes 56 ₁₁ to 56 _(NM),58. Each midamble sequence 54 ₁ to 54 _(N) is associated with a set ofchannelization codes 56 ₁₁ to 56 _(NM), as illustrated in FIG. 5. Thesets may contain only a single channelization code, which is a one toone mapping of midambles to channelization codes. A burst transmitted bythe base station 12 ₁ with a channelization code of a midamble's set isformatted with that midamble sequence, 60, 62. To illustrate, if a burstwith channelization code 21 was sent, midamble sequence 2 is used forthat burst.

At the UE receiver 28, after channel estimation, the transmittedmidamble sequences are detected by a midamble sequence detection device48, 64. Based on the detected midambles, a logic block 45, utilizing themidamble to channelization code mapping 49, determines the set ofpossible channelization codes. A channelization code detection device 50determines the received channelization codes based on the determination,66. If a one midamble code to one channelization code mapping is used,the logic block 45 determines the channelization codes. As a result, fora one to one mapping, the channelization code detection device 50 is notused. The MUD device 46 uses the determined channelization codes and thechannel response for the midamble sequences associated with thechannelization codes to detect the data from all the bursts, 68.

One channelization code detection device 50 is shown in FIG. 6. Matchedfilters 82 ₁ to 82 _(M) are matched to the possible channelization codesand associated channel responses as determined by the logic block 45.Since only the possible channelization codes need to be checked, thenumber of matched filters 82 ₁ to 82 _(M) is greatly reduced, reducingthe complexity and improving the performance of the receiver 28. Thepower of the soft symbols produced by each matched filter 82 ₁ to 82_(M) is measured by corresponding power measurement devices 84 ₁ to 84_(M). The comparator 80 determines the received channelization codesbased on the power measurement for each channel. If the number oftransmitted channelization codes is known, the comparator 80 selectsthat number of channels with the highest measured power. Otherwise, thecomparator 80 compares each channel's power level to a threshold todetermine the transmitted channelization codes.

To aid in identifying channelization codes, channelization codeinformation, such as transmitted channelization codes or a number oftransmitted channelization codes, may be signaled to the UE 14 ₁. Thesignaled information can be used in conjunction withchannelization/midamble code mapping or when mapping is not used. Theadditional channelization code information will increase the accuracy indetermining the active channelization codes at the UE receiver 28. Onesuch signal would be a layer one signal, where the midamble code ormidamble code shift is associated with the information. The midambledetection device 48 determines the received midamble code(s) and thelogic block 45 recovers the channelization code information using thedetermined midamble codes. Using the recovered information, thechannelization code detection device 50 uses the recovered informationto aid in the channelization code determination. Another approachsignals channelization code information using a layer 2/3 signal. Thesignal is generated by the network circuitry. The layer 2/3 signal canbe used in conjunction with layer one signals or with themidamble/channelization code mapping.

1. A method for selecting midamble codes in a wireless time divisionduplex communication system, the method comprising: assigning at leastone channelization code to a user equipment; providing mapping betweenmidamble codes and channelization codes, where at least one midamblecode is mapped to a plurality of channelization codes having a samespreading factor; and selecting the midamble code mapped to the at leastone channelization code to the user equipment; and forming acommunication burst with the assigned at least one channelization codeand the selected midamble code.
 2. A wireless time division duplex basestation, the base station comprising: means for assigning at least onechannelization code to a user equipment; a mapping between midamblecodes and channelization codes, where at least one midamble code ismapped to a plurality of channelization codes having a same spreadingfactor; and means for selecting the midamble code mapped to the at leastone channelization code to the user equipment; and means for forming acommunication burst with the assigned at least one channelization codeand the selected midamble code.
 3. A wireless time division duplex basestation, the base station comprising: a spreading and modulation devicefor forming a communication burst with an assigned channelization codeand a selected midamble code, the assigned channelization code beingassigned to a user equipment, a mapping between midamble codes andchannelization codes is provided where at least one midamble code ismapped to a plurality of channelization codes having a same spreadingfactor; and the selected midamble code is mapped to the channelizationcode.